The present invention relates to computer storage systems. In particular it relates to storage systems which have a tip directed close or in contact to the storage medium by which bit-writing and bit-reading is enforced.
It is a general aim for the computer industry to increase the storage density of the storage systems being used by computers. Every new technology, however, which is a good candidate to replace today""s storage methods should offer long-term perspectives in order to give room for continued improvements within this new technology over a couple of years because with a fundamental change of storage technology the computer industry would have to undertake remarkable investments in order to adapt existing production machines or to replace existing machines by new ones for any technical purpose involved with said new technology.
Thus, the consequence for further development of storage systems is that any new technique with better storage area density should have a long-term potential for further scaling, desirably down to the nanometer or even atomic scale.
The only available tool known today that is simple and yet provides these very long term perspectives is a nanometer sharp tip. Such tips are used in every atomic force microscope (AFM) and scanning tunneling microscope (STM) for imaging and structuring down to the atomic scale. The simple tip is a very reliable tool that concentrates on one functionality: the ultimate local confinement of interaction.
In recent years, AFM thermomechanical recording in polymer storage media has undergone extensive modifications mainly with respect to the integration of sensors and heaters designed to enhance simplicity and to increase data rate and storage density. Using heater cantilevers, thermomechanical recording at 400 Gb/in2 storage density and data rates of a few Mb/s for reading and 100 kb/s for writing have been demonstrated as it is described in G. Binnig, M. Despont, U. Drechsler, W. Haeberle, M. Lutwyche, P. Vettiger, H. J. Mamin, B. W. Chui, and T. W. Kenny, xe2x80x9cUltrahigh-density atomic force microscopy data storage with erase capabilityxe2x80x9d, Applied Physics Letter, Volume 74, Number 9, Mar. 1, 1999, pp 1329-1331.
Such prior art thermomechanical writing is a combination of applying a local force by the cantilever/tip to the polymer layer and softening it by local heating. By applying sufficient heat an indentation can be formed into the storage medium for writing a bit which can be read back with the same tip, by the fact that the lever is bent when it is moved into the indentation and the electrical resistance of a sensing circuit is changed therewith.
While writing data or bits, the heat transfer from the tip to the polymer through the small contact area is initially very poor and improves as the contact area increases. This means the tip must be heated to a relatively high temperature, about 400xc2x0 C., to initiate the melting process. Once melting has commenced, the tip is pressed into the polymer, which increases the heat transfer to the polymer, increases the volume of melted polymer, and hence increases the bit size. After melting has started and the contact area has increased, the heating power available for generating the indentations increases by at least ten times to become 2% or more of the total heating power. With this highly nonlinear heat-transfer mechanism it is very difficult to achieve small tip penetration and hence small bit sizes as well as to control and reproduce the thermomechanical writing process.
A further problem in this polymer/tip approach is that each bit-writing process effectuates a structural change in the storage medium, i.e. the above mentioned indentation, which is subjected to mechanical wear on the one hand and which is the cause for mechanical wear of the sensing tip on the other hand. Thus, a large number of subsequent reading processes reduces the bit quality.
A further disadvantage in said approach is that single bits can not be erased. Instead, they can only be erased in blocks of about 10 Mbit.
During the practical utilization of a storage medium, however, a byte-selective erase process is often preferred.
Both of the last mentioned problems, i.e., mechanical wear and insufficient local resolution during bit-erasing are caused by the same feature, i.e. the fact, that the bit-writing process is coupled to a structural change in the medium which appears at the medium surface.
A different approach to write into and read from a storage medium without any change of the medium""s surface is the magneto-optical approach as it is disclosed for example in DE 197 07 052. A laser beam is used for heating up the storage medium locally, and bits are written in the heated region preferably, when the coercive field of the storage area is low. This approach, however, is limited in area resolution as the laser spot transferring the heat into the storage medium has at least the size of half of the laser wave length, or can be reduced further only marginally by superposition of e.g., the kernels of two laser spots having each a reduced power. Thus, with said approach bit-sizes of about 200-400 xcexcm can be achieved. Those huge bit-sizes, however lead to storage densities which are not able to be tolerated within the ambitious aim of increasing storage densities up to values of several 100 Gb/in2.
It is an objective of the present invention to provide a storage system which does not suffer from mechanical wear and which allows for storage densities of several 100 Gb/in2.
In order to write bits or data in digital form into the storage medium the basic concepts of the present invention comprise using a magnetizable storage medium, expose it to a magnetic field coupled externally to the storage medium, and during writing to concurrently apply heat locally in bit size dimension in order to let the external magnetic field become locally larger than the coercive field at the location where heat is applied.
According to the present invention this is achieved by providing a storage system comprising a magnetizable storage medium, a source of an external magnetic field, and at least one write head, but preferably an array of write heads. Each write head has a small dimensioned tip with a resistive path or a resistive loop such that when driving a current through it heat is exerted onto the location where the bit or data storage is intended to be performed.
It should be noted that the inventional concept includes using a magnetic coil, or any other magnetic source, even a permanent magnetic layer to provide the external magnetic field.
Data or bit writing is provided by heating the storage medium locally when the current flows through the resistive path and the local temperature approaches or reaches the Curie temperature of the storage medium, or the compensation temperature, whereby the source of said magnetic field produces a magnetic field which is higher than the coercive field at a given temperature. Thus, a bit can be written into the storage medium.
In particular and according to a preferred embodiment of the inventional storage system, it is proposed to provide a one- or two-dimensional array of cantilever tips each of which serves as a heat source when activated by a current flowing through a resistive path within said tip. This produces the necessary temperature at the small storage medium location where the bit writing is intended to approach the Curie temperature or the compensation temperature of the magnetic material.
Basically, bits can be generated with or without direct contact between tip and storage medium. If there is no contact, which might be a significant advantage compared to thermo-mechanical bit writing, a soft medium can advantageously be taken as a heat guide between tip and storage medium. If there is a contact then a lubricant may be used for wear reduction and for increasing the heat transfer.
The storage medium is proposed to be formed as a thin plate the surfaces of which are covered with some protection layer.
During bit writing when the Curie, or the compensation temperature, is reached a coil arranged in parallel below or above the storage medium produces a magnetic field effective to larger portions of the medium, including in particular the same medium location heated by the tip heat. The area covered by the magnetic field should be larger than the intended bit size in order to assure a sufficient magnetic effect. The magnetic field is higher than the coercive field and thus, a bit is written with a magnetization along the direction of this external field. In this way, stable bits can be written.
Preferred materials for bit writing with the described concept are materials with a large perpendicular magnetic anisotropy. The perpendicular magnetic anisotropy has to be larger than shape anisotropy in order to stabilize the magnetization along the direction perpendicular to the surface plane. Materials fulfilling this criterion are rare earth transition metal alloys such as TbFe, GdCo, DyFe, or mixtures, i.e., compositions thereof. These materials are typically ferrimagnets with sublattices of oppositely oriented magnetization which do not exactly compensate at ambient temperature. The exact amount of e.g. Tb vs. Fe is chosen such that either the temperature where the sublattice magnetizations compensate, i.e., the compensation temperature, is at a temperature best suited for the writing process, or such that the temperature at which the materials loose their ferromagnetism, i.e., the Curie temperature, is not too far above the writing temperature.
Typical materials also include ternary compounds (e.g. TbFeCo) or even quaternary or higher (e.g. GdTbDyFeCo) in variable composition. Most of these materials have the largest perpendicular anisotropy in an amorphous state. Another material class are the garnets, which all contain oxygen as a nonmetal element. Examples are Y3Fe5O12, or BaFe12O19.
In all these materials the rare earth element is the most relevant for the large perpendicular anisotropy. A completely different class exploits a property of many ultrathin magnetic films. Ultrathin films often have a perpendicular surface anisotropy which stabilizes the perpendicular magnetization, even when they do not contain rare earths. Examples are Co films grown on Au(111), Pt(111), Pd(111), or Fe films grown on Cu(001). Depending on the exact film thickness (typically 1 to 5 atomic layers, i.e.  less than 1 nm) the anisotropy and other magnetic properties can by tailored to have optimum material properties for the proposed writing process.
Multilayers of these materials (such as Co/Pt/Co/Pt/Co/Pt . . . ) can stabilize the materials and make them more robust against external influences such as corrosion, etc. Alloys of CoPt with varying composition are a variant of the concept of using optimized magnetic properties existing at surfaces in bulk materials.
Depending on the selected material, the thickness of the magnetizable layer can be adapted to the respective aimed and focused properties of the storage medium.
According to a further preferred aspect of the present invention the before mentioned approach can be combined with the xe2x80x98Millipedexe2x80x99 technique which is disclosed in U.S. Pat. No. 5,835,477, titled xe2x80x98Mass-storage Applications of Local Probe Arraysxe2x80x99, issued Nov. 10, 1998. For more detailed information on structure and functionality, as well as on fabrication know-how of the basic elements of the millipede arrangement it should be referred thereto.
Said millipede is a two-dimensional arrangement of such cantilever tips, i.e. a local probe array, and the storage medium comprises array fields which correspond in distance to the distances between said millipede tips. A read or a write process comprising more than the millipede area is achieved by moving the millipede in parallel to the medium surface in a x- or y-direction in order to find a new position having a starting point for the whole array. This feature is referred to herein as global tracking feature for the whole array.
The approach of writing a bit with a millipede arrangement has the advantage that no pre-patterning of the media is necessary and that therefore the problems with respect to positioning the cantilever array at predefined locations can be circumvented because the array is self-aligned as the location of the bits during read back is determined by the position of the tips during writing. Thus, the small spatial tolerances required to read back the intended bit can easily be achieved compared to a case where pre-patterning is performed and only a fraction of the two-dimensional bit array can be retrieved successfully.
A further advantage according to the present invention is that storage materials can be used which are not limited to presently employed magneto-optic media because the issue of a large magneto-optic response at a certain read out wave length does not exist. Thus, materials can be used with optimized magnetic and structural properties such as high magnetization and resistance to corrosion, e.g. Co/Pt multilayers or alloys, as was already mentioned above.
It should be stressed that according to the present invention bit sizes of about 20 nanometer and smaller can be realized whereas the only comparable prior art method for magnetically writing bits is the magneto-optical method where a bit size of about 200-300 nanometer can be realized only. Thus, the inventional concept combines the reliability of magnetic bit writing with the advantages achieved from using a tip for localizing the bit area.